Osteogenic proteins were defined originally as an activity present in mammalian bone extracts, presumably active during growth and natural bone healing, capable of inducing a developmental cascade leading to cartilage and endochondral bone accumulation when implanted in vivo. This developmental cascade includes mesenchymal cell recruitment and proliferation, progenitor cell differentiation, cartilage calcification, vascular invasion, bone formation, remodeling and marrow differentiation (Reddi, Collagen Rel. Res., 1, pp. 209-26 (1981)).
The factors in bone matrix that induce endochondral bone differentiation can be dissociatively extracted and reconstituted with inactive collagenous matrix to restore full bone inductive activity (Reddi, Proc. Natl. Acad. Sci. USA, 78, pp. 7599-7603 (1981)). This provides an experimental method for assaying protein extracts for their ability to induce endochondral bone formation in vivo. Using this reconstitution assay, a variety of related osteogenic proteins have been isolated from several mammalian species that are capable of inducing bone and cartilage formation in cross-species implants (Sampath and Reddi, Proc. Natl. Acad. Sci. USA, 80, pp. 6591-95 (1983)). The active factor or factors that promote this activity have been referred to in the literature most commonly as bone morphogenetic proteins (BMPs) and osteogenic proteins (OPs).
Osteogenic and bone morphogenetic proteins represent a family of structurally and functionally related morphogenic proteins belonging to the Transforming Growth Factor-Beta (TGF-.beta.) superfamily (see below). The TGF-.beta. superfamily, in turn, represents a large number of evolutionarily conserved proteins with diverse activities involved in growth, differentiation and tissue morphogenesis and repair. BMPs and osteogenic proteins, as members of the TGF-.beta. superfamily, are expressed as secretory polypeptide precursors which share a highly conserved bioactive cysteine domain located near their C-termini. Another feature of many of the BMP family proteins is their propensity to form homo- and heterodimers.
Many morphogenic proteins belonging to the BMP family have now been described. Some have been isolated using purification techniques coupled with bioassays such as the one described above. Others have been identified and cloned by virtue of DNA sequence homologies within conserved regions that are common to the BMP family. These homologs are referred to as consecutively-numbered BMPs whether or not they have demonstrable osteogenic activity. Using an alternative approach, synthetic OPs having osteogenic activity have been designed using amino acid consensus sequences derived from sequence comparisons between naturally-derived OPs and BMPs (see below; Oppermann et al., U.S. Pat. No. 5,324,819).
While several of the earliest members of the BMP family were osteogenic proteins identified by virtue of their ability to induce new cartilage and bone, the search for BMP-related genes and gene products in a variety of species has revealed new morphogenic proteins, some of which have different or additional tissue-inductive capabilities. For example, BMP-12 and BMP-13 (identified by DNA sequence homology) reportedly induce tendon/ligament-like tissue formation in vivo (WO 95/16035). Several BMPs can induce neuronal cell proliferation and promote axon regeneration (WO 95/05846). And some BMPs that were originally isolated on the basis of their osteogenic activity also have neural inductive properties (Liem et al., Cell, 82, pp. 969-79 (1995)). It thus appears that osteogenic proteins and other BMPs may have a variety of potential tissue inductive capabilities whose final expression may depend on a complex set of developmental and environmental cues. These osteogenic, BMP and BMP-related proteins are referred to herein collectively as morphogenic proteins.
The activities described above, and other as yet undiscovered tissue inductive properties of the morphogenic proteins belonging to the BMP family are expected to be useful for promoting tissue regeneration in patients with traumas caused, for example, by injuries or degenerative disorders. Implantable osteogenic devices comprising mammalian osteogenic protein for promoting bone healing and regeneration have been described (see, e.g., Oppermann et al., U.S. Pat. No. 5,354,557). Some osteogenic devices comprise osteogenic protein dispersed in porous, biocompatible matrices. These naturally-derived or synthetic matrices typically allow osteogenic protein to diffuse out of the matrix into the implantation site and permit influx and efflux of cells. Osteogenic protein induces the progenitor cells to differentiate and proliferate. Progenitor cells may migrate into the matrix and differentiated cells can move out of the porous matrix into the implant site. Osteogenic cells may also utilize the matrix as a physical scaffold for osteoconduction. Similar devices have been described for delivering BMPs for tendon/ligament-like and neural tissue regeneration (see below). Osteogenic protein-coated prosthetic devices which enhance the bond strength between the prosthesis and existing bone have also been described (Rueger et al., U.S. Pat. No. 5,344,654, incorporated herein by reference).
The availability of large amounts of purified and highly active morphogenic proteins would revolutionize orthopedic medicine, certain types of plastic surgery, dental and various periodontal and craniofacial reconstructive procedures, and procedures generally involving bone, cartilage, tendon, ligament and neural regeneration. Many of the mammalian OP- and BMP-encoding genes are now cloned and may be recombinantly expressed as active homo- and heterodimeric proteins in a variety of host systems, including bacteria. The ability to recombinantly produce active forms of morphogenic proteins such as OPs and BMPs, including variants and mutants with increased bioactivities (see below), make potential therapeutic treatments using morphogenic proteins feasible.
Given the large number of potential therapeutic uses for morphogenic proteins in treating a variety of different tissues and tissue-types, there is a need for highly active forms of morphogenic proteins. It would thus be desirable to increase the tissue inductive properties of morphogenic proteins. With increased tissue inductive activity, treatment with a morphogenic protein, even on large scales, could induce tissue formation more rapidly, or tissue induction could be achieved using reduced morphogenic protein concentrations.